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Uranium and Dirty Bombs

At a 1 June 2004 press conference, the Department of Justice announced that Jose Padilla, the "Dirty Bomber," had planned to make a dirty bomb, or Radiological Dispersal Device, using uranium. Even though a following Associated Press article pointed out that uranium in not particularly radioactive and would not be a useful dirty bomb material ["Much less to Padilla 'dirty bomb' than meets feds' eyes, scientists say," Charles J. Hanley, AP Special Correspondent, 9 June 2004.], uranium-as-dirty-bomb-material reports continue in the press, even in prestigious newspapers.

An important measure of any radioactive substance is how quickly the radioactive atoms disintegrate, or "decay." This rate is usually described by the half-life, or the length of time that half the atoms will take, on average, to decay. Different radioactive elements can have widely different half lives, some shorter than a second, others billions of years. Dirty bombs will use materials of intermediate half lives, measured in years.

Even tiny amounts of a material with a half life of days will be intensely radioactive because the material is disintegrating so fast. It will not, however, be a good dirty bomb material because too much will decay while the bomb is being built. Even if the bomb disperses the material, just waiting several days for the material to decay naturally will allow people to return to the contaminated areas. At the opposite extreme, radioactive materials with extremely long half lives just do not decay quickly enough to cause much radiation. Looked at another way, to get dangerous levels of radiation requires huge quantities of material if the material decays very slowly. Uranium is more than 99% U-238, which has a half life of almost five billion years. The rest is U-235, which has a half life of almost a billion years.

Radiation is often measured in "Curies," equivalent to a gram of radium. A one Curie radiation source is dangerously radioactive and requires special handling. One Curie of Cs-137, with a half life of 30 years, contains just over a hundredth of a gram of cesium, less than a pinch of salt, but to get a Curie of uranium requires 3057 kilograms, or over three tons. In a Scientific American article on dirty bombs, FAS calculated the effects of a 3500 Curie release in Manhattan, showing that such an attack would contaminate most of the island. The table below shows the weights required of various radioactive materials to make 1.0 and 3500 Curies. Note that a 3500 Curie uranium dirty bomb would require thousands of tons of uranium. Vaporizing this and getting it into the air would require many times that weight of explosive. Uranium is simply not a good radioactive bomb material.

Radioactive substances are also distinguished by the decay process that they undergo. There are three possible products from spontaneous decay: alpha and beta particles, and gamma rays. Alpha particles are relatively slow and heavy when they emerge from the nucleus. They have low penetrating power, and clothes are sufficient to keep them from penetrating to the body. In fact, alpha particles do not penetrate the surface of the skin, when exposed from outside the body. Beta particles are high energy electrons. Like alpha particles, they cannot penetrate very far and are of greatest concern if ingested. Gamma rays, on the other hand, are not particles but rays, like radio waves or light waves, except they are much higher in energy. Gamma rays can pass through the walls of buildings into the body and cause immediate damage. Cesium-137 and cobalt-60, for example, emit powerful, ionizing gamma-rays. Almost all of the energy of U-235 and U-238 is released in alpha particles. Because uranium decays via alpha particles, it is harmful primarily when it is ingested. Once inside the body, uranium's alpha particles can ionize molecules and damage DNA.

In addition to being radioactive, uranium is poisonous. So there will be some health effect of spreading uranium around, just as a bomber could mix his explosive with asbestos or mercury or lead to make clean up more difficult and expensive. But that does not meet anyone's definition of a "dirty" bomb.

Isotope

Primary Radiation

Weight of 1 Curie

Weight of 3500 Curies

Americum-241

alpha

0.30 grams

1.05 kilograms

Cobalt-60

gamma

0.91 milligrams

3.20 grams

Californium-252

alpha

1.88 milligrams

6.59 grams

Cesium-137

beta/gamma

11.8 milligrams

41.40 grams

Iridium-192

beta/gamma

0.11 milligrams

0.39 grams

Plutonium-239

alpha

16.8 grams

58.70 kilograms

Plutonium-240

alpha

4.54 grams

15.90 kilograms

Radium-226

alpha

1.04 grams

3.64 kilograms

Strontium-90

beta

7.17 milligrams

25.1 grams

Uranium-238

alpha

3.06 tons

10.7 thousand tons

Depleted Uranium (0.2%)

alpha

2.10 tons

7.35 thousand tons

Natural Uranium (0.70%)

alpha

1.50 tons

5.24 thousand tons

Commercial Uranium (5%)

alpha

0.42 tons

1.46 thousand tons

(There are 28 grams in an ounce and 2.2 pounds in a kilogram. Metric tons are about 10% more than US "short" tons.)